The present invention relates to improvements in the electron microscope, electron beam lithography apparatus, ion microscope, secondary ion mass spectrometer or similar charged particle beam apparatus employing a finely converged electron or ion beam. Still more particularly, the invention relates to structural improvements in a charged particle beam optical system associated with the charged particle beam apparatus.
Conventionally, the charged particle beam optical system used in the above-mentioned charged particle beam apparatus utilizes as its objective lens a magnetic lens made of a yoke 14 and an exciting coil 15 (FIGS. 4 and 12), or an Einzel lens constituted by three electrodes (FIGS. 5 and 13). The objective lens is used to converge a charged particle beam on a sample. (Some aspects of the magnetic lens and the Einzel lens are illustratively disclosed in Japanese Patent Laid-Open Nos. 63-160144 and 63-252347, respectively.)
A charged particle beam generator in the charged particle beam apparatus generally comprises an electrostatic lens that draws a charged particle beam from a charged particle source and accelerates (or decelerates) the beam with a predetermined level of energy for convergence onto an object. The electrostatic lens is illustratively constituted by a three-electrode lens containing three electrodes 19, 20 and 21 in FIGS. 12 and 13. The electrostatic lens and objective lens in the charged particle beam generator for beam convergence need to be optically aligned by mechanical or electrical alignment means. A typical prior art electrical alignment means for precise optical axis alignment of the charged particle beam allows the axial displacement between lenses to be corrected by an alignment coil 23' in a space of ground potential, e.g., between the three-electrode lens and the magnetic lens of FIG. 12, or between the three-electrode lens and the Einzel lens of FIG. 13. The charged particle beam is deflected and the astigmatism correction thereof achieved using a deflecting coil 5' and a stigma coil 24' in a space of ground potential under the electrostatic lens (i.e., on the sample side) constituting the charged particle beam generator. (This aspect of the charged particle beam apparatus is illustratively disclosed in Japanese Patent Laid-Open No. 63-67743.)
The charged particle beam apparatus such as the electron microscope utilizes a finely converged charged particle beam for observation of the sample surface. With such apparatus, what primarily determines the performance thereof (e.g., resolution) is the spot diameter of the charged particle beam converged by the objective lens onto the sample. How small the spot diameter can be depends on how the optical aberration of the objective lens is minimized. To minimize the optical aberration of the objective lens requires positioning the sample as close to the principal plane of the objective lens as possible so that the focal distance of the objective lens will be minimized. As shown in FIG. 4, the conventional magnetic lens has its principal plane 11 located in a magnetic field within a gap H of the yoke 14. Therefore, to position a sample 6 close to the principal plane 11 requires putting the sample 6 somehow into the narrow gap H of the yoke 14. This can be a major constraint on sample location where a sample with a large surface area is to be observed or where the same sample is to be observed or measured concurrently by other observing or measuring instruments connected to the charged particle beam apparatus. And as depicted in FIG. 5, the conventional Einzel lens has the principal plane 11 thereof always located therewithin regardless of a positive or negative voltage being applied to the center electrode thereof. This prevents the sample 6 from getting placed sufficiently close to the principal plane 11 of the lens. The structural constraint necessarily promotes the optical aberration of the lens.
Where the conventional charged particle beam apparatus uses an electrostatic lens or the combination of an electrostatic lens with a magnetic lens to converge a charged particle beam onto a sample, it is important to minimize the optical aberration involved so as to produce a beam spot with the lowest defocusing value on the sample surface. On the electrostatic lens, the axial displacement between electrodes is a major factor that increases the optical aberration. Since there are limits to the precision in machining and assembling the electrodes, there is no avoiding some degree of inter-electrode axis displacement. Conventionally, the axial displacement between the lenses constituting the charged particle beam apparatus (i.e., between electrostatic and magnetic lens or between two electrostatic lenses) is corrected using an electrical means. That is, the electrical means aligns the charged particle beam within the space of ground potential between the lenses. Conventionally, however, the axial displacement of the electrodes constituting the electrostatic lens is not precisely corrected by the electrical means. Typically, the electrodes of the electrostatic lens are mechanically adjusted from outside the apparatus (i.e., from the atmospheric pressure side) for axial alignment. But problems remain with the precision of mechanical adjustment and the repeatability thereof for a number of reasons. One reason is a complicated apparatus structure necessitated by insulation of the voltage applied to the electrodes as well as by sealing of the vacuum space. Another reason for the above problems is the mechanical backlash that remains.
To minimize chromatic aberration and spherical aberration requires positioning the lens as close to the sample as possible. This requirement is conventionally met by installing a charged particle beam deflection means or an astigmatism correction means under (i.e., on the sample side) the electrostatic lens (e.g., three-electrode lens) constituting the charged particle beam generator. Because this structure prevents the sample from getting positioned sufficiently close to the electrostatic lens which is part of the charged particle beam generator, it is necessary to provide another objective lens under the electrostatic lens. The result is a very complicated electronic optical system that keeps the charged particle beam apparatus from being manufactured small.